Method for improving properties of articles



Nov. 17, 1964 A. R. BOBROWSKY 3,156,974-

METHOD FOR IMPROVING PROPERTIES OF ARTICLES Filed Feb. 10, 1961 IN VEN TOR.

ALFRED R. BOBROWSK ATTORNEYS United States Patent 3,156,974 METHOD FGR IMPROVING PROPERTIES OF ARTICLES Alfred R. Bobrowsky, Florham Park, NJ, assignor to Engelhard Industries, Inc, Newark, NJ, at corporation of Delaware Filed Feb. 10, 1961, Ser. No. 88,420 11 Claims. (Cl. 29-423) This invention relates to improving properties of articles, for instance metallic articles, by a method involving the repeated application of high hydrostatic pressure.

In accordance with the present invention, the article to be treated, for instance a metallic article, is encased in a jacket having a larger compressibility than the article, and a high hydrostatic pressure is applied to the resulting assembly. After the specified time of treatment, the pressure is released, i.e. reduced or removed. The application of high hydrostatic pressure to the encased material and the subsequent release of pressure is then repeated one or more times, preferably a plurality of times followed by removal of the jacket from the article, for instance by melting. By reason of the repeated application of the high hydrostatic pressure and the release of pressure, considerable hardening and improvement in other properties is achieved in the article. In addition, a material increase in the number of dislocations with attendant improvement in certain properties of the article hereafter discussed, is achieved by the repeated application of high hydrostatic pressure.

Preferably, the application of the high hydrostatic pressure and the release of pressure is repeated 3-10 times.

The improvements achieved by the present invention not achieved by other types of Working are: (1) size and shape of the articles or specimens are not greatly altered so that frequently specimen can be used immediately after processing, without further machining; (2) the articles can be subjected to greater shear stress while under hydrostatic pressure prior to catastrophic failure such as rupture or excessive deformation than without the presence of hydrostatic pressure, and hence appreciably greater amounts of hardening are achieved with the same number of cycles of Working than by competitive processes; (3) articles or materials too brittle to be Worked by any usual technique, for instance with dies, may be altered in properties by this technique; (4) dies are not required to maintain definite shape; and (5) hardening and other improvement of properties is achieved throughout the entirety or substantial entirety of the article rather than only on the surface.

Each cycle of application of high hydrostatic pressure is preferably of long duration of at least about one minute, more preferably from about one hour-one week. Appiication and subsequent release of pressure is preferably at a slow and controlled rate, preferably a rate of pressure a'iplication or build-up of about 30 kilobars per minute to 30 kilobars per seconds and asimilar rate for release of such pressure. However, it may be applied rapidly and released even more rapidly. On the contrary, hardening techniques of the prior art using hammers (impact or impulsive machinery) or involving the use of rolls or presses with or without dies do not apply continual strain to the specimens for appreciable periods of time, and hence do not achieve the improved results attained by the present invention. In the case of hammers, continual strainis applied for durations of the order of a few milli- 3,l55,974 Patented Nov. 17, 1964 "ice seconds, in the case of rolls usually for a few hundredths or tenths of a second, and in the case of presses for a few seconds.

The method of this invention gives good results in the hardening of specimens or articles of irregular shape or without symmetry, for instance cams, bolts or certain electron-tube components. It is not infrequently difficult to harden irregularly shaped articles with the use of dies because of protruding or outwardly extending elemerits, arms, or other protruding members possessed by these articles. However, by encasing the irregularly shaped articles in the jacket and employing repeated applications of high hydrostatic pressure in accordance with this invention, the article can be readily hardened to the extent desired. The method also gives good results in the hardening of regular shape or symmetrical articles such as, for example, symmetrical cylinders, spheres, cubes, etc. The hardening of symmetrical long cylinders is especially valuable because this shape frequently occurs in materials of construction, design of machine elements, certain components of electron tubes and other portions of devices. In FIGURE 1 of the accompanying drawings, which is a longitudinal section through the enoasing jacket, cam 16 is encased in jacket 11 of a low-melting bismuth-containing alloy prior to application of high hydrostatic pressure. FIGURE 2, also a longitudinal section through the jacket, shows a regularly shaped long right circular cylinder 12 with enlarged end portions 13 and 3.4 all encased in jacket 15 of the low-melting bismuth-containing alloy prior to the pressure application.

FIGURE 3, also a longitudinal section through the en-' casing jacket, shows a regularly shaped long right circular cylinder 16 with roughened end portions 17 and 18, all encased in jacket 19 of the low-melting bismuth-containing alloy prior to application of the high hydrostatic pressure.

In addition to the hardening brought about by the process of this invention, other improvements in the material resulting from the treatment include a strengthening, a great increase in creep resistance, an increase in stressrupture life, and improvements in electrical and magnetic properties of metals, alloys and semiconductors. Such changes are believed due to an increase in the number of the materials dislocations, and a favorable change in their location and arrangement.

Dislocations are imperfections or defects in crystalline material and are described in detail in the book entitled Dislocations and Plastic Flow of Crystals, by A. H. Cottrell, Oxford, Clarendon Press, 1953. According to theory, the number and form of such dislocations effects certain properties of materials such as hardness, creep resistance and electrical conductivity, but does not effect certain other properties such as for example, modulus of elasticity, to any great extent.

The article or specimen to be treated is encased in a material that is more compressible than the article and preferably one that also undergoes an abrupt densification or reduction in volume at a predetermined elevated pressure. Materials which undergo an abrupt densification or reduction in volume at a predetermined elevated pressure and which can be used for the jacket of this invention are, for example, bismuth, low-melting bismuth-containing alloys and cerium. Many other materials are known which also undergo such densification or reduction in volume. In some cases this densification is a result of a polymorphic transformation from one crystal structure to another closer packed crystal structure. In other cases it is believed this abrupt densification is due to the shifting of an outer electron to an electron shell which is closer to the atomic nucleus, and in still other cases, abrupt densification occurs under circumstances which are not Well understood. In Table I set forth below, the pressure levels at which abrupt densification takes place for three elements are set forth:

Table 1 Element: Pressure-atmospheres Bismuth 25,000 Cerium 7,600 Barium 60,000

Thechanges in volume are relatively large with that for bismuth being about 9 percent and that for barium about 3 percent. The jacket, which is of non-elastomeric material, need not be in the form of a pure chemical element. Preferably, the material of the jacket is a low-melting bismuth-containing alloy, for instance the eutectic alloy of bismuth, lead and tin melting at 203 F. By reason of the jacket undergoing an abrupt densification under high pressure, the encased article is subjected to an intensified pressure materially higher than that applied to the jacket. The jacket should be of such material that placing it on the specimen or removing it does not harm the specimen.

Typical means of accomplishing the jacketing operation are:

(a) Casting a jacket around the article;

(b) Placing two halves of a precast jacket around the article and welding tight the seams of the jacket;

() Casting the article inside the jacket; and

(d) Placing the article in a through hole in a precast jacket, and mounting collars at the ends of the article I in juxtaposition to the end surfaces of the jacket.

Hydrostatic pressures employed in the method of this invention may be within the range of about 1,000l00,000 atmospheres, preferably about 20,0003 0,000 atmospheres.

In applying the hydrostatic pressure, the jacketed article to be hardened is placed within the pressure cylinder or chamber of a conventional and well known hydrostatic pressure apparatus. One such apparatus comprises a hydraulic arrangement disclosed in my copending patent application Serial No. 819,942, filed June 12, 1959 now abandoned, The upper pressure limit of this apparatus might be lower than is desired, but the use of the preferred jacket of this invention which undergoes an abrupt densification achieves higher pressures than can be attained by use of the conventional apparatus.

When hydrostatic pressure is applied to the jacket of the assembly, the jacket tends to reduce in volume to a greater extentthan the specimen. Since the compressions of the diameter and length of the jacket are not aliected equally by the presence of the specimen, the result is that the radial pressure on the specimen is not identical with the longitudinal stress, in general. On removal of the pressure, the converse behavior occurs.

Inasmuch as the stresses in the jacket are different in diiferent directions, shear stresses are present that may exceed the elastic limit of the jacket at the given hydrostatic pressure. Elastic limit is defined as the maximum strain or stress that can be applied to a material Without causing a permanent change in dimension or shape of the specimen, when the stresses on the boundary of the specimen have been removed.

One cycle of applying pressure and either removing or reducing pressure constitutes a working of the jacketed article. Repeated application of such cycles induces hardening, increase in strength, and a material increase in the content of imperfections such as dislocations in the crystal line material, this hardening, strength increase and increase in the content of dislocations being surprisingly high.

The articles to be hardened in accordance with the present invention can be fabricated of metals or nonmetals. Exemplary of the metals are the ferrous alloys,

cobalt-base high-temperature alloys and nickels alloys.

The valuable efiects and results achieved by the method of this invention are manifest in materials intended for use for structural, structural-at-elevated-temperature, wearing, bearing, supporting, electrical, magnetic and other purposes, as well as in devices wherein components of this type are utilized and in processes in which there are utilized devices in which components of this type are utilized. 1

it will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

What is claimed is: V

1. A method for hardening articles, which comprises encasing the article having a given compressibility in a solid'non-elastomeric jacket having a larger compressibility, applying a high hydrostatic pressure between about 20,000 and 30,000 atmospheres to the resulting assembly, releasing the pressure, repeating the application of high hydrostatic pressure and the subsequent release of pressure at least one time to produce hardening throughout the substantial entirety of the article, and removing the jacket from the thus-treated article.

2. A method for hardening metallic articles, which comprises encasing the metallic article having a given compressibility in a solid, non-elastorneric jacket having a larger compressibility, applying a high hydrostatic pressure between about 20,000 and 30,000 atmospheres to the resulting assembly, releasing the pressure, repeating the application of high hydrostatic pressure and the subsequent release of pressure at least on time to produce hardening throughout the substantial entirety of the article and increase the number of dislocations therein, and removing the jacket from the thus-treated article.

3. The method of claim 2 wherein the jacket is of a material of the group consisting of cerium, bismuth, and low-melting bismuth-containing alloys.

4. The method of claim 2 wherein the application of high hydrostatic pressure and the subsequent release of pressure is repeated 3-l0 times.

5. The method of claim 2 wherein the metallic article is of irregular shape.

6. A method-for. improving properties of metallic articles which comprises encasing the metallic article having a given compressibility in a solid, non-elastomeric jacket having a larger compressibility, applying a high hydrostatic pressure between about 20,000 and 30,000 atmospheres-to the resulting assembly, releasing the pressure, repeating the application of high hydrostatic pressure and the subsequent release of pressure at least one time and removing the jacket from the thus-treated article. 7. A cyclic method for hardening articles, which comprises encasing the article having a given compressibility in a solid, non-elastomeric jacket having a larger compressibility, applying a high hydrostatic pressure between about 20,000 and 30,000 atmospheres to the resulting assembly, releasing the pressure, repeating the application of pressure and subsequent release of pressure at least one time to produce hardening throughout the substantial entirety of the article, each cycle of hydrostatic pressure application being of long duration, and removing the jacket from the thus-treated article.

8. The method of claim 7 wherein each cycle of hydrostatic pressure application is of a duration from about one hour to one week.

'9. The method of claim 7 wherein the application and subsequent release of pressure is at a slow and controlled rate.

10. The method of claim 7 wherein the pressure is applied and subsequently released at a rate of about 30 kilobars per minute to 30 kilobars per 10 seconds.

. 11. A cyclic method for hardening crystalline metallic articles, which comprises encasing the crystalline article having a given compressibility in a solid, non-elastomeric jacket having a larger compressibility, applying a high hydrostatic pressure between about 20,000 and 30,000 atmospheres to the resulting assembly, releasing the pressure, repeating the application of pressure and subsequent release of pressure a plurality of times, each cycle of hydrostatic pressure application being of a duration from about one hour to one week to harden the article and to increase the number of dislocations therein, and removing the jacket from the thus-treated article.

References Cited in the file of this patent UNITED STATES PATENTS Shearman July 31, 1866 Langenberg Dec. 10, 1932 Flowers Aug. 14, 1945 Stoddard April 15, 1952 MeKenna et al Aug. 11, 1953 MacLeod Mar. 1, 1955 Staba Oct. 30, 1962 

1. A METHOD FOR HARDENING ARTICLES, WHICH COMPRISES ENCASING THE ARTICLE HAVING A GIVEN COMPRESSIBILITY IN A SOLID NON-ELASTOMERIC JACKET HAVING A LARGER COMPRESSIBILITY, APPLYING A HIGH HYDROSTATIC PRESSURE BETWEEN ABOUT 20,000 AND 30,000 ATMOSPHERES TO THE RESULTING ASSEMBLY, RELEASING THE PRESSURE, REPEATING THE APPLICATION OF HIGH 